The invention is based on a videophone system. In the case of such a system, also called a videophone, the amount of light falling from the objective onto the target must be controllable. It is known for this purpose for an aperture to be provided, as in the case of a photographic apparatus, which is open to a greater or lesser width.
In order to reduce the costs of such a mechanically adjustable aperture, it is also known for a fixed aperture opening to be provided which can be opened or closed electronically only completely, like a shutter. The amount of light falling onto the target is controlled or regulated by this shutter being opened for a time of about 0.5-1 ms in each case, which is short in comparison with the frame duration. The amount of light falling onto the target is thus varied electronically in this case by varying the opening time of the shutter
Such videophone systems normally use data reduction or data compression for the signal transmission. This comprises, for example, not transmitting the contents of an image in each case but only the differences between successive images. It has been found that faults occurred as a result of the data reduction in the case of a videophone system having a controlled exposure time of the type described. It has been found in particular that macroblocks were generated in the codec even in the case of a constant image, that is to say without any differences between successive images, even though there were no changes in the image between successive images. The use of data reduction for a camera having the said controlled exposure time thus appeared impossible or problematic.